Selecting a Brush-Commutated DC Motor

BASIC PARAMETERS

Permanent magnet direct current (DC) motors convert electrical energy into mechanical energy through the interaction of two magnetic fields. One field is produced by a permanent magnet assembly; the other field is produced by an electrical current flowing in the motor windings. The relationship between these two fields results in a torque that tends to rotate the rotor. As the rotor turns, the current in the windings is commutated, or switched, to produce a continuous torque output.

Brush DC motors can be operated over a wide range of voltages, speeds, and loads. Output power for a brush DC motor is a product of speed and torque; input power is a product of the applied voltage and motor current. The first step in motor selection is to decide if you are going to need a gearbox or not. This will typically depend on your maximum required load speed. A good rule of thumb might be to use a gearmotor if your maximum speeds will be below 1000 RPM, and use only a motor if your maximum speeds will be above 1000 RPM.

If you are going to use a gearbox, start by selecting one that meets the torque requirements of your application. Gearboxes are usually rated by their maximum allowable output (load) torque. Once you have chosen a gearbox type, the appropriate ratio must be selected. Determine the ratio by dividing the maximum acceptable input speed to the gearbox by the maximum desired output (load) speed, then choosing the closest available ratio. Acceptable gearbox input speeds vary, but are typically on the order of 6000 RPM. Calculate the motor speed and torque requirements using the following equations:

WM = WL x N and TM = TL / (N x n) where WM = Motor Output Speed

WL = Load Speed

N = Gear Ratio

TM = Motor Output Torque

TL = Load Torque

n = Gearbox Efficiency

Once the motor requirements have been determined, choose a motor type and frame size capable of producing the required motor torque. For continuous operation, select a motor with a continuous torque rating greater than that of the required torque. For intermittent operation with a sufficiently short on-time, select a motor with a continuous torque greater than that of the rms value of the required torque.

Motor manufacturers will provide continuous torque ratings for their motors under certain operating conditions, including a specified ambient temperature (often 25 degrees C. or 40 degrees C.) and thermal resistance (dependent on whether a heat sink is utilized.) Take care to read the fine print when comparing continuous torque ratings as they may need to be adjusted if these assumptions do not match your actual operating conditions.

After a frame size has been selected, the proper winding needs to be specified. Generally, voltage and torque will be known values, and speed and current will need to be determined. The best winding choice will be that which comes closest to providing the desired speed and current draw given the supply voltage and load torque. The governing motor equations to determine speed and current follow:

W = (VS - I x Rmt) / KE and I = TL / KT + INL where W = Speed

VS = Supply Voltage

I = Current

Rmt = Motor Terminal Resistance

KE = Back-EMF Constant

T = Load Torque

KT = Torque Constant

INL = No-Load Current

While these equations are suitable for most applications, it is important to realize that they are only the basic formula and do not take into account thermal considerations. Motor heating will alter some of the parameters in these calculations, including resistance, torque constant, and back-emf constant. Accounting for these effects adds significantly more complexity to the process. Finally, when going through any calculations, make sure to maintain consistency among units of measure.

COMPONENT FEATURES

"Off-the-shelf" brush-commutated DC motors are the exception, rather than the rule, and they are frequently customized to meet specific design and performance criteria for an application. Among those components typically specified:

Optical Encoders: Since closed loop servo applications require velocity and/or position feedback, common motor options include incremental optical encoders, which supply accurate position, velocity, acceleration, and direction feedback for precision motion control. Encoders can be added to any motor or gearmotor with wires or side-exiting power terminals and can be metal-housed or open air. They can be factory-mounted or prepared for mounting in the final stages of end-product assembly. Encoders are usually specified with either two- or three-channel, TTLcompatible quadrature outputs. The maximum frequency, which limits the maximum operational speed, is typically 100 KHz. In a three-channel unit, the third channel provides an index signal or pulse once per revolution of the codewheel.

Another encoder option, the rotary pulse indicator (RPI), is a single-channel unit with open-collector or TTL-compatible outputs. RPIs are low-cost solutions for appliance applications that need 120 counts per revolution or less without direction-sensing capabilities.

Shafts: The shaft of any motor can be customized with a flat, journal, cross hole, keyway, slot, groove, gear, or pulley. These options can be combined to meet application requirements. As examples, a cross hole can allow a pulley to be pinned to the shaft, or a journal can include a groove. A variety of other combinations are possible. Shaft material can be customized from the standard 416 Stainless steel to other grades, such as 303 and 316 Stainless with different Hardness ratings. Standard and common optional shaft diameters include a variety of sizes from 4mm to 8 mm and from 5/32-inch to 3/8-inch.

Gearheads: Gearheads increase output torque and decrease speed. These functions and their efficiency vary with different models and applications. For most applications a spur gearhead is flexible enough to meet specific torque, noise, and cost requirements. Standard spur gearheads feature sintered nickel-steel gears, which provide moderate power handling with average audible noise. The sintering process allows for close tolerances (AGMA Q7-8) at a low cost. The sintered gear functions as a lubrication holder and helps dampen sound. When more strength is required, a hybrid cluster (an assembly of a cut-steel pinion and a sintered gear) or precisioncut steel gears can be chosen. Other gearhead options include planetary gearheads for lower backlash and much higher torque or Delrin (moldable polymer) gears that produce less noise than sintered gears.

Wire and Cable Assemblies: Custom wire and cable assembly options are designed to speed motor installation and boost component reliability. Almost any connector style and wire type can be specified for motors, gearmotors, and encoders.

EMI/RFI Suppression Components: A number of cast and stamped component solutions have been developed to reduce the amount of electrical noise generated by a motor. For low-frequency RFI (typically below 30 MHz) capacitors are generally effective, and there is an inverse relationship between the value of the capacitor and the attenuated noise frequency. Capacitors installed by the motor manufacturer enable strategic placement inside the motor frame for optimum filtering as close to the noise source as possible. For high-frequency noise (generally above 30 MHz) ferrite beads can help reduce RFI. A combination of ferrite beads and capacitors provides the most effective suppression by creating a low-pass LC filter that is inductive-capacitive at low frequencies and dissipative at high frequencies.

Mounting for each component may vary from slipping ferrite beads over wires to soldering chokes near the motor terminals, depending on the best solution for the application.

Brakes: Developed as a safety and energy-saving feature, rear-mounted power-off and power-on electro-magnetic brakes prevent a motor or gearmotor from rotating freely. Brakes typically are offered for 16 and 40 oz-in static torques and 12, 24, 28, 48, and 90 VDC operation, although other voltages, including 120 VAC, are available. A power-off brake stops a motor when power is removed and releases the motor when power is reapplied. In low-duty applications, the brake saves energy by maintaining a known motor position without power. An added safety feature is that should power be lost while the motor is lifting an object by pulley or lead screw, the brake will lock the motor and prevent the object from falling. A power-on brake holds the motor in place upon application of power and releases the motor when power is removed.

DEALING WITH EXTREMES

When brush-commutated DC motors are used to drive gears or pulleys, avoid excessive side loads. These can push a motor to an extreme and lead to motor failure. If side loads will be present, ball bearings are usually recommended. Environmental conditions will impact, too, on effective brush DC motor operation and performance. For example, the moisture in the air acts as a lubricant and, where humidity is low, the resulting lower lubrication will accelerate brush wear and shorten motor life. (Special brushes are designed to solve this problem.)

AVOIDING PITFALLS

Know the proper rating of the motor for an application and recognize and understand the importance of continuous operation vs. duty cycle.

Do not press fit components on a motor's shaft (in any direction) without proper support at the other end of the shaft. This action could lead to motor failure.

Do not apply adhesives or other foreign material directly to shafts that could contaminate the bearings. These could negatively affect performance. If such materials are to be applied, it is generally advised to apply them to the component to be secured to the shaft to reduce the chance of contamination.

Consult with your motor manufacturer before, during, and after a motor is specified for an application.

STANDARDS AND REGULATIONS

NEMA publications represent the most relevant sources for standards relating to traditional motor products and devices. Other standards include ANSI and IEC for rotating machinery, as well as IEEE standards for motor-related test procedures. A "CE" designation assures compliance with appropriate standards for those products used in the European marketplace. In addition to product standards, a set of qualityoriented standards applies to motor suppliers. Those manufacturers that have achieved ISO certification demonstrate documented adherence to procedures and operations consistent with international quality standards.

MOTOR FAILURE MODES

The primary cause for failure of brush-commutated DC motors over time is ongoing brush wear. The traditional method for mounting copper or silver graphite brushes in motor assemblies has been to solder the brushes onto standard cantilever springs to enable the required constant contact with the commutator. This conventional spring design, however, carries inherent drawbacks as force levels diminish over time, and motor failure can result.

The problem can be overcome by housing the brushes within a specially designed cartridge and utilizing torsion springs to ensure desired even force over the life of a motor. The cartridge, which fits into the motor base, consists of a two-piece, hightemperature plastic snap-together assembly in which each of two brushes is seated securely within its own specially constructed slot. This design effectively restricts the brushes to traveling in a track in a desired linear motion.

The cartridge design further provides for an ideal region of pressure (6-8 lbs. psi) for the brushes to withstand the detrimental effects of mechanical wear. Other typical causes that can result in motor failure include motor overloading, contamination of the armature, and electrical or mechanical malfunctions. There are many others, depending on motor design, operating parameters, and in-use service and safeguards.

COST SAVINGS

Users can save money (and headaches) at the outset by partnering with a quality motor manufacturer from the very beginnings of the design stage. This will minimize (and likely eliminate) costly mistakes and ensure that a motor performs as intended and required in an application.

This early involvement also can open a window to available motor features and options, which could help initially to reduce labor and material-handling costs for the customer and provide for easier motor installation.